Nerves pass from one body area to another through channels made of connective tissue and/or bone. In these narrow passages, they can get trapped due to anatomic abnormalities, ganglion cysts, muscle or connective tissue hypertrophy, tumours, trauma or iatrogenic mishaps. Nearly all nerves can be affected. The clinical presentation is pain, paraesthesia, sensory and motor power loss. The specific clinical features will depend on the affected nerve and on the chronicity, severity, speed and mechanism of compression. Its incidence is higher under some occupations and is some systemic conditions: diabetes mellitus, hypothyroidism, acromegaly, alcoholism, oedema and inflammatory diseases. The diagnosis is suspected with the clinical presentation and provocative clinical test, being confirmed with electrodiagnostic and/or ultrasonographic studies. Magnetic Resonance Studies (MRI) rule out ganglion cysts or tumours. Conservative medical treatment is often sufficient. In refractory ones, surgical decompression should be performed before nerve damage and muscle atrophy are irreversible. The ‘double crash’ syndrome happens when a peripheral nerve is compressed at more than one point along its trajectory. In cases with marked muscle atrophy, a ‘supercharge end‐to‐side’ nerve transfer can be added to the decompression. After decompression in those few cases with refractory pain, a nerve neurostimulator can be applied.
- entrapment neuropathy
- compression neuropathy
- carpal tunnel syndrome
- cubital tunnel syndrome
- meralgia paraesthetica
- cheiralgia paraesthetica
- peroneal nerve entrapment
- ulnar tunnel syndrome
- radial tunnel syndrome
- tarsal tunnel syndrome
Nerves pass from one body area or cavity to another through holes and channels made of connective tissue and/or a bone channel, be it total (mental nerve) or partial (carpal tunnel). In these narrow passages, they can get trapped and/or injured due to congenital anatomical abnormalities, muscle or connective tissue hypertrophy, ganglion cysts, tumours, trauma or iatrogenic mishaps. Nearly all nerves can suffer an entrapment syndrome. Not all have the same incidence, some being very common (i.e. carpal tunnel)  and some exceedingly rare (i.e. tarsal tunnel) .
Entrapment neuropathies results in pain, paraesthesia and muscle power loss in the distribution of a peripheral nerve. With time muscle atrophy and skin trophic changes will appear. The clinical presentation will depend on the specific affected nerve, the chronicity, severity, speed and mechanism of compression [3, 4].
Nerve entrapment incidence is higher under some systemic conditions: diabetes mellitus, hypothyroidism, acromegaly, chronic alcoholism, extensive oedema and systemic inflammatory diseases . Some occupations are associated with specific peripheral nerve entrapment syndromes. For example, occupations requiring repetitive wrist or finger movements or handling of vibrating tools have a higher incidence of carpal tunnel syndrome (CTS) [5–7].
Clinical presentation and provocative tests will suggest a diagnosis  confirmed or not with electrodiagnostic or ultrasonographic studies [4, 8]. Moreover, electrodiagnostic studies are also helpful to stage the severity and to rule out other confounding conditions (i.e. carpal tunnel and C7 radiculopathy, peroneal nerve compression vs. L5 radiculopathy) [3, 9] or generalized diseases (i.e. diabetic peripheral neuropathy) [9, 10]. MRI studies often show changes, ganglion cysts or tumours [11, 12] but the ultrasonography is less costly and more easily available .
Conservative treatment is sufficient in many cases (i.e. Saturday night palsy) but otherwise surgical decompression should be considered before irreversible peripheral nerve damage and muscle atrophy are established [3, 4, 8].
A nerve can be compressed at more than one single point, exacerbating the effects [14, 15]. This is called the ‘
In cases with advanced muscle atrophy, a
CTS is the most frequent entrapment syndrome, followed by meralgia paraesthetica and UN in the elbow. Decompression is always the treatment, removing the fibrous band, muscle or benign lesion causing the entrapment. After decompression, cases with refractory pain can undergo a nerve neurostimulator to block the pain transmission.
2. Upper extremity entrapment syndromes
2.1. Carpal tunnel syndrome (CTS)
This tunnel is formed by the ‘U’ of the carpal bones closed by the transverse carpal ligament. It is the most frequent entrapment neuropathy and one of the most common surgical conditions [17, 18]. Its estimated prevalence is 2% in men and 3% in women [17, 18], affecting a 3.72% of the USA population .
Idiopathic forms are due to a connective tissue proliferation of the flexor tendons synovium . Some medical conditions predispose to its development: diabetes mellitus [20, 21], acromegaly , obesity , pregnancy , amyloidosis , hypothyroidism , rheumatoid arthritis , chronic kidney disease  and haemodialysis . Its incidence is higher in occupations requiring repetitive finger and wrist movements , handling of vibrating tools [5–7] or repetitive blows with the palm of the hand (carpenters, sculptors) , but not with keyboard use . It affects 30% of diabetics with polyneuropathy and 14% without it . In pregnancy, it is most common in the third trimester .
Patients notice pain, numbness and tingling in the first three fingers of the hand. Initially symptoms are intermittent but become permanent with time, worsening with activity and at night . Symptoms wake patients up at dawn, making them shake the affected hand to get rid of the symptoms (the so called
Sensory deficits affect the thumb, index and middle fingers and spare the thenar eminence , but 20% of clear‐cut CTS show no sensory abnormalities . Because the palmar cutaneous branch for the thenar eminence branches off the MN a few centimetres before the carpal tunnel the sensation of this area is normal in CTS. If this sensation is impaired pre‐operatively it indicates proximal MN compression  while if damaged is only seen post‐operatively it indicates iatrogenic injury.
Entrapment of this branch is possible but exceedingly rare .
Symptoms are usually bilateral but predominate in one hand.
The diagnosis is suspected by the symptoms and provocative manoeuvres (Phalen test (Figure 1A) and the Tinel and the carpal compression signs) . The Phalen test indicates advanced disease , having a 75% sensitivity and a 47% specificity . Electrodiagnostic studies confirm the diagnosis, rule out confounding conditions and stage the disease , with an 85% sensitivity and a 95% specificity . Symptoms do not always correlate with electrodiagnostic findings. Ultrasonography is also useful . Due to its higher costs, MRI is not used regularly .
Up to 20% of CTS cases improve with conservative treatments [44, 45]. Night‐time wrist splints help 60% of patients but many eventually need an operation [46, 47]. Local corticosteroid injections can provide relief but often temporary . Surgical decompression is the only proven long‐term lasting relief [40, 49]. Any concomitant systemic disease predisposing to CTS should be treated at once although decompression is usually needed nonetheless . The surgical procedure entails complete transverse carpal ligament section to decompress the MN. Local, regional or general anaesthesia are options, but local is faster and more cost effective [50, 51]. Open field (Figures 1F and G) or endoscopy has a similar time out of work, but the latter MN damage is more frequent [52–54]. Retinaculotome decompression is similar to endoscopy but with less time and cost requirements  (Figures 1H and I). Re‐operation is indicated in failure or recurrence. Incomplete decompression either at the distal carpal ligament or at the proximal antebrachial fascia is a frequent finding , but sometimes there is a thick scar tissue recreating the transverse carpal ligament and fixing the MN .
It is the MN compression as it passes through the pronator
2.3. Anterior interosseous syndrome
It is due to compression of this purely motor branch of the MN . It induces a mild vague forearm pain accompanied by paresis or complete paralysis of the FPL and flexor
2.4. Ulnar nerve compression at the elbow
Cubital tunnel syndrome (CubTS) is the entrapment of the UN at the elbow . It is the most common site of UN entrapment and the second most common in the upper extremity nerve . Its estimated incidence is 25 new cases/100,000 inhabitants/year [67, 68], affecting males more often than females [68–72]. It is more common in jobs with constant leaning on the elbow (i.e. book keepers, drivers resting the elbow on the window frame) , gripping tools (gardeners, farmers, builders) , professional motorbike runners, cyclist [75, 76], repetitive elbow flexoextension [73, 74] and in floor cleaners [73, 77]. It is also more frequent in some systemic disorders like diabetes mellitus , acromegaly , rheumatoid arthritis  or amyloidosis . CTS and CubTS in the same arm is not a rare finding [81–83].
Its clinical presentation consists of pain, sensory loss, paraesthesias, motor weakness and muscle atrophy at the forearm ulnar side and fourth and fifth fingers . If untreated, it can lead to lack of sensation and muscle power, as well as pain and clumsiness in the affected hand . Patients often complain of a dull pain at the elbow with shock‐like sensations with any mild pressure or blow on this area. Some patients notice no sensory symptoms because of progressive weakness in the fourth and fifth fingers accompanied by muscle atrophy of the hand intrinsic muscles (Figures 2E and F) . Symptoms get worse with activity and on flexing the elbow.
On clinical examination, the fifth finger remains in abduction due to weakness of the fourth palmar interosseous muscle (
The most common site of UN entrapment is the retroepicondylar groove followed by the cubital tunnel 1.5–3 cm distal to the epicondyle . About 40% of the cases are idiopathic . The causes of compression are a bulky triceps muscle , the
The diagnosis is based on the symptoms. Electrodiagnostic studies confirm the diagnosis and rule out other medical conditions (i.e. C8 radiculopathy) .
Some patients may improve with conservative measures like avoiding external elbow pressure, using a night time split to keep the elbow extended or stopping any occupational activity that might be causing the disease. If that is not enough or the patient presents with muscle weakness and atrophy, a surgical decompression is indicated.
The techniques for UN decompression at the elbow are medial epicondylectomy,
Pre‐operative and intra‐operative electrophysiological inching studies have found that the compression point is at or immediately proximal to the cubital tunnel [87, 111–113], less often at the Osborne’s arcade but not proximally at the intermuscular septum . Others with endoscopic assistance have reported no nerve constriction beyond 4 cm distally or proximally to the retroepicondylar tunnel . So, extensive proximal decompression seems futile [100, 114]. Unsatisfactory results have been related to concomitant undiagnosed CTS or to weight gain .
2.5. Ulnar nerve compression at the hand
It is an uncommon site for UN entrapment (Figure 5A). Depending on the exact point of compression, it can be classified into five types [88, 115, 116]. In type I, the compression is proximal to Guyon’s canal with involvement of the superficial sensory, hypothenar motor, as well as deep motor branch. In type II, the compression is inside the canal and only the superficial sensory branch is affected. In type III, the compression is distal to the sensory branch with involvement of the hypothenar and deep motor branch proximal to the branch for the ADM. In type IV, the compression is distal to the superficial sensory and the hypothenar branch, so only the deep motor branch is affected. In type V, there is compression to the deep motor branch just proximal to the adductor
Usually there is the antecedent of an acute trauma  or chronic compression (cyclists) . In other cases, there is a structural lesion in the area compressing the nerve, most commonly a ganglion cyst . In cases of repetitive compression (i.e. cyclists), removal of the offending activity can be tried. If there is a lesion it has to be removed before irreversible UN damage develops (Figures 5B–E) .
2.6. Radial nerve (RN) entrapment syndromes
Its entrapment points are  at the spiral groove by the intermuscular septum between the triceps and brachialis (BaM) muscles, at the proximal forearm by the ligament of Frohse (posterior
2.7. Suprascapular nerve entrapment
It can be trapped at the suprascapular and spinoglenoid notches where the nerve is fixed by ligaments in a bony canal . The first symptom is pain localized in the posterior aspect of the shoulder that gets worse with activity, when lying on the affected area, or by shoulder adduction crossing the midline with the extended arm . The weakness and atrophy of the supra and infraspinatus muscles induce paresis of shoulder abduction and external rotation.
On clinical examination, the affected shoulder is lower than the healthy one and the scapular muscles are atrophied (Figure 8A). The patient has difficulty raising the outstretched arm above the horizontal (Figures 8B and C).
Its treatment is surgical with section of the ligament that closes the suprascapular notch at the superior aspect of the scapula. It can be done open field  (Figures 8D and E) or endoscopically  with similar outcomes.
2.8. Thoracic outlet syndrome
There is pain in the inner aspect of the arm and forearm, sometimes reaching the fourth and fifth fingers . This pain gets worse when lifting the arm above the horizontal . Sometimes there is associated hand muscle atrophy . Claw hand deformity can be present in the long protracted cases . The neurogenic type has an incidence of one case per million inhabitants . It can be due to hypertrophy of some muscles at the root of the arm (typical of ceiling painters or swimmers) or to the existence of a cervical rib or fibrous ligament at the same point .
In case of poor response to conservative treatment, surgical decompression is in order. The two possibilities are the transaxillary removal of the first rib  or supraclavicular scalenectomy  (Figures 9A–D). This depends on the causative mechanism and the surgeon’s preferences but the supraclavicular approach offers a better chance of solving any causative abnormality .
3. Lower limb entrapment syndromes
3.1. Meralgia paraesthetica
The femoral cutaneous nerve is a purely sensory nerve which runs usually medial to the ASIS. The name meralgia paraesthetica comes from the Greek,
The clinical presentation in the absence of motor signs helps to make the diagnosis. Electrodiagnostic studies can rule out confounding conditions [144, 156], but ultrasonography is very useful, particularly in obese patients [156, 157].
Initially, the treatment is to remove the compressing agent and/or lose weight. If insufficient, the area can be infiltrated with a local anaesthetic agent and corticosteroids . The rebel cases require surgical treatment with nerve decompression (Figures 10C–E) or neurectomy .
3.2. Peroneal nerve entrapment
It is the most common lower limb entrapment neuropathy . It is a mixed nerve that runs at the fibular head, reaching the anterior compartment of the leg distal to the knee . At that level, it lies between the skin and bone. This makes it very sensitive to trauma or pressure, particularly in bedridden lean patients . It can also be due to mass lesions (i.e. ganglion cyst of the tibiofibular joint) or associated with systemic diseases (diabetes and vasculitis) . It is more frequent in occupations requiring people to squat for long periods of time (strawberry pickers, farm workers and carpet layers)  or that sit crossing their legs .
The clinical presentation is pain at the fibular head and loss of strength in dorsiflexion (Figure 11A), which causes the foot to drag when walking. The patient notices foot slap with steppage gait and wearing the tip of the shoe as well as a sensory loss on the dorsal aspect of the foot between the first and second toe .
3.3. Anterior tarsal tunnel
It is the entrapment of the deep peroneal nerve. The clinical presentation is pain in the dorsum of the foot associated with sensory loss in the first foot web space . The treatment is initially conservative, but surgical decompression with extensive fascial opening might be needed .
3.4. Tarsal tunnel syndrome
It is due to compression of the posterior tibial nerve at the tarsal tunnel behind the foot medial malleolus. It is very uncommon. Many cases are idiopathic (20–46%) . Contributing factors are ankle sprain and fracture, tight‐fitting foot wear and space occupying lesions . The clinical presentation is pain, paraesthesia and numbness in the sole of the foot. This symptoms get worse on standing, walking and at night time . The sensory loss affects the sole of the foot sparing the heel, supplied by the calcaneal branch . The diagnosis is identified with the clinical presentation. Electrodiagnostic studies can be useful to rule out confounding medical conditions . Ultrasonography  and MRI  can rule out associated space occupying lesions.
Its initial treatment is rest and anti‐inflammatories, but if there is no improvement or relapse after an initial response, surgical decompression may be necessary. This can be done endoscopically , but for a good decompression, especially in the distal part, an open approach gives better results  (Figures 12A–D).
3.5. Piriformis syndrome
It is a very rare disorder in which
The conditions associated with this syndrome are sitting for extended periods of time, sitting with a large wallet in the rear pocket, repeated forward movements, running, bicycling, stiff sacroiliac joints, foot overpronation, Morton’s toe (the second toe is longer than the first one) and after a fall on the buttocks [177–179]. Approximately 50% of the cases are caused by trauma and the rest are spontaneous .
The symptoms are sciatica‐like pain. Pain starts in the gluteal area and may travel through the back of the thigh and calf up to sole the foot. Patients might experience tingling, numbness, burning sensation and weakness. The sciatic pain aggravates with sitting or with activities that press the piriformis against the sciatica nerve, such as running, cycling or hose riding .
The diagnosis is usually made through physical examination. Certain tests may elicit sciatica nerve pain indicating the presence of the syndrome, especially internal rotation of the hip with the knee in full extension.
On MRI examination, it is possible to see the sciatic nerve with oedema when crossing under the piriformis muscle.
Conservative treatment is initially recommended. Alternate ice and heat treatment may provide relief. Ultrasound penetrates deep into the muscle alleviating the sciatica nerve pain. Stretching exercises to target the piriformis, hamstrings and hip muscles, will help increase the range of motion and decrease the sciatic nerve pain.
If all these treatments prove unsuccessful, injection of botulinum toxin in the piriformis muscle  under CT or MRI guidance can be attempted. In the case of failure, surgical decompression removing the piriformis muscle or the offending fibrous band could be indicated [179, 181] (Figures 13A–E). The results are inconsistent.
3.6. Pudendal nerve entrapment
It induces pain in the genital and sometimes gluteal areas . The pain worsens with local pressure, sitting, defecating, and urinating and with sexual intercourse . It is constant, intense and burning. The cause can be local pressure induced by repeated cycling  or by horse riding. The problem is that most patients are diagnosed late. Once suspected, it can be confirmed with electrodiagnostic studies . When conservative treatments  fail, surgical decompression should be considered  (Figures 14A–E). The results are often poor, at times because patients are diagnosed much too late due to lack of awareness in the medical world.
Nerve entrapments syndromes are more frequent than currently thought. Their awareness is essential to diagnose and treat the patient on time. Although almost any nerve can suffer an entrapment syndrome, some are more common than others. The most frequent is CTS, followed by meralgia paraesthetica and ulnar nerve entrapment at the elbow. The clinical presentation is pain, paraesthesia and muscle power loss in the distribution of the affected nerve. Many cases are idiopathic, but others are induced by internal or external compressing mechanisms. Some systemic conditions are associated with an increased incidence of these syndromes. The clinical presentation together with the electrodiagnostic studies help in the diagnosis. The ultrasonography and the MRI are also helpful but not used so regularly.
In many cases, conservative medical treatment is sufficient. When it is not, surgical decompression has to be performed.
Open and endoscopic approaches are available. In each case, we will have to see which shows better outcomes. A few cases with persistent pain after surgical decompression might benefit from peripheral nerve neurostimulation.
We thank the Department of Human Anatomy and Embryology of the Faculty of Medicine of the University of Valencia, particularly to the laboratory curators Lucia and Carmina and to Dr. Tomás Hernández Gil de Tejada, and to all personnel of the
Appendices and nomenclatures
|AIN||Anterior interosseous syndrome|
|ASIS||Anterior superior iliac spine|
|CubTS||Cubital tunnel syndrome|
|CTS||Carpal tunnel syndrome|
|ECRB||Extensor carpi radialis brevis|
|ECRL||Extensor carpi radialis longus|
|FDS||Flexor digitorum superficialis|
|FDP||Flexor digitorum profundus|
|FPL||Flexor pollicis longus|
|PIN||Posterior interosseous nerve (PIN)|
|PQ||Pronator quadratus muscle|
|PTM||Pronator teres muscle|